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Inclusion Criteria:

* Male or female participants aged 8 to 17.5 years (inclusive) at the time of enrollment
* Diagnosis of major depressive disorder (MDD) according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5)
* Evidence of clinically significant symptoms of depression as indicated by a T score\> 60 on the Children's Depression Inventory 2 (CDI-2) parent form at prescreening
* Evidence of clinically significant symptoms of depression in the mild to moderate range as indicated by a score between 40 and 70 on the Children's Depression Rating Scale--Revised (CDRS-R) at screening
* Subjects and their caregivers must be English-speaking and have a level of understanding sufficient to communicate intelligently with the investigator and study coordinator, and to cooperate with all tests and examinations required by the protocol.
* Subject and their caregiver must be willing and able to comply with all study procedures.
* Each subject and his/her authorized parent/guardian must understand the nature of the study. The subject's authorized parent/guardian must sign an informed consent document, and the subject must sign an informed assent document.
* Subject must be able to swallow pills.
* Subject must have access to a computer with a camera, speaker, microphone, and internet connection.
* Subjects must be willing to refrain from treatment changes during the study protocol.

Exclusion Criteria:

* Recent history or active clinically significant manifestations of infectious, metabolic, hepatic, renal, hematological, pulmonary, cardiovascular, gastrointestinal, musculoskeletal, dermatological, urogenital, neurological, endocrinological or eyes, ears, nose, and throat disorders, or any other acute or chronic condition that, in the Investigator's opinion, would limit the participant's ability to complete or participate in this clinical study.
* History of sensitivity to thiamine
* Severe allergies or multiple adverse drug reactions.
* History of nutritional deficiencies other than thiamine deficiency that could interfere with the study protocol or confound results.
* DSM-V diagnosis of substance use disorder, abuse or dependence.
* Judged clinically to be at serious suicidal risk for Columbia-Suicide Severity Rating Scale (C-SSRS) score ≥4.
* Severe symptoms of depression as indicated by CDRS-R score \>70.
* Current diagnosis of bipolar disorder.
* Current diagnosis of schizophrenia.
* Current diagnosis or symptoms of psychosis.
* Previous or current diagnosis of Intellectual Disability.
* Participants who are on stable pharmacotherapy will be included as long as the medication has been stable for at least 6 weeks prior to screening and will continue without change for the duration of the study. Participants on pharmacotherapy that has not been stable for at least 6 weeks will be excluded.
* Participants who are in stable psychotherapy will be included as long as they have started psychotherapy at least 3 months prior to screening and will continue therapy without change for the duration of the study. Participants receiving psychotherapy that has not been stable for at least 6 weeks will be excluded.
* Concomitant treatment or treatment within the last month with neuroleptics, antipsychotics, anticonvulsants, sedatives or hypnotics.
* More than two antidepressant treatment failures at an adequate dose and duration (at least 6 weeks) in the current major depressive episode.
* Having started psychotherapy (including cognitive behavioral therapy \[CBT\]) less than 3 months prior to Screening
* Pregnancy: Female participants who are pregnant or breastfeeding will be excluded to avoid risks to the child and ensure safety in the use of thiamine.